The mid-day roost does not make sense to me. Consider, the sun heats the earth, the earth heats the air - creating upward moving masses of air called ‘Doldrums’. The migratory birds find these doldrums, circle within then and ‘glide’ higher and higher - then leave the doldrum and ‘glide’ to the next one, and repeat.
Gliding is far easier and more efficient than flapping your wings for both altitude and distance - why work, when you can rest?
The night time flocking is a safety precaution, from predators. There is safety in numbers, so if a predator attacks, one ‘sqwalk’ can alert the entire flock; rather than being a lone prey.
The early-morning activity, IMHO is a feeding frenzy - to catch the insects while they are still a little lethargic (until the sun warms them up). Easy meal time - and a meal to last them for the day’s flight.
Just a farm boy’s observations.
All of which is true—but the fact remains that a large part of migratory birds, if not the larger part, cover their distances at night. Easiest by far to notice are mallards and Canada Geese. It’s not something that has been well explained by ornithologists, many of whom chalk it up to birds observing the stars for flight patterns, which I don’t buy much. But still they do it.